The limited supplies of natural gas (methane) in the United States, together with its great usefulness, have provided the necessary incentive for the discovery and development of techniques to produce synthetic natural gas (SNG) by a reaction known as methanation. The methanation reaction generally involves the conversion of synthesis gas to methane and water in the presence of a suitable catalyst. Synthesis gas is a mixture of CO and hydrogen and can be produced by the gasification of coal with steam and oxygen. Suitable catalysts for methanation are described in the prior art and include iron, nickel and ruthenium, among others. The Bureau of Mines Report of Investigation 5137 entitled "Synthesis of Methane" by Murrat Greyson et al. (July 1955) reports that nickel is superior to iron and that the techniques of catalyst preparation determine to a large extent the process life of the nickel catalyst.
The nickel catalysts investigated by the Bureau and others are typically prepared by precipitating nickel salts such as nickel nitrate onto various supports such as alumina or kieselguhr. In addition to poor aging characteristics, prior art nickel catalysts suffer from their tendency to promote undesired side reactions such as the disproportionation of the CO to CO.sub.2 and the formation of carbon either by the decomposition of CO or the formation of higher molecular weight hydrocarbons which eventually deposit and form coke.
A superior methanation catalyst has now been discovered which tends to avoid coke and the disproportionation of CO to CO.sub.2 at high temperatures and in addition tends to maintain substantially complete conversion of CO in synthesis gas over longer periods of time at any given nickel level and temperature.
An improved methanation reaction is accomplished in accordance with the invention by contacting CO, CO.sub.2, or mixtures of these carbon oxides and hydrogen wherein the molar ratio of hydrogen to the carbon oxides is at least 2:1 under methanation conditions in the presence of a catalyst comprising a crystalline layered complex metal silicate characterized as having repeating units having the structural formula: EQU 8 (1 -x) Ni.sup.a + xRu.sub.b ].sub.n (OH).sub.4 Si.sub.2 O.sub.5.sup.. wH.sub.2 O
where x is a number from 0 to 1, this number expressing the atomic fraction of the metals nickel and ruthenium, a is the valence of nickel, b is the valence of ruthenium, n is a number equal in value to that defined by the ratio EQU 6/[ a(1-x) + bx]
and w is a number ranging from 0 to 4.
The improved methanation catalyst for use in the process of this invention is a known layered complex metal silicate wherein the metal is selected from nickel, ruthenium, or mixtures of these metals. These layered complex metal silicates and their methods of preparation are described, for example, in U.S. Pat. No. 3,729,429 to Robson issued Apr. 24, 1973. The specification of the Robson patent is incorporated herein by reference for the purpose of providing a fuller description of the catalyst and a method of preparing the catalyst. It is realized that the materials described by Robson encompass many complex metal silicates while only the nickel and ruthenium or mixed nickel-ruthenium complex metal silicates are claimed in this specification as useful materials to promote the methanation reaction. Robson in his specification describes his metal silicates as useful catalytic agents in hydrocarbon conversion reactions. Illustrative of such reactions are aromatization, isomerization, hydroisomerization, cracking, hydrocracking, polymerization, alkylation, dealkylation, hydrogenation and dehydrogenation, desulfurization, denitrogenation and reforming (see Col. 3, lines 14-18 of the '429 Robson patent). Nowhere does Robson teach or indicate that his materials, especially the nickel or ruthenium forms, are useful for the synthesis as contrasted with the conversion of hydrocarbons and in particular the synthesis of methane.
More specifically, the catalyst used to promote the desired methanation reaction in accordance with this invention is a crystalline layered complex metal silicate composition characterized as having repeating units having the structural formula EQU [(1-x)Ni.sup.a - xRu.sup.b ].sub.n (OH).sub.4 Si.sub.2 O.sub.5.wH.sub.2 O
where x is a number from 0 to 1, this number expressing the atomic fraction of the metals nickel and ruthenium, a is the valence of nickel, b is the valence of ruthenium, n is a number equal in value to that defined by the ratio EQU 6/[ a(1-x) + bx]
and w is a number ranging from 0 to 4.
The preferred metal silicate is where x in the above formula equals 0. The resulting material is a nickel chrysotile, and naturally occurring nickel chrysotile is known as garnierite.
Thus either naturally occurring nickel chrysotile can be employed to promote the subject reaction, or, more preferably, a synthetically prepared nickel chrysotile can be employed. One suitable method of preparing the catalysts of this invention is, as noted above, by the technique of Robson in U.S. Pat. No. 3,729,429. As noted by Robson at the top of Column 4, Ni.sub.3 (OH).sub.4 Si.sub.2 O.sub.5 (garnierite) is found in nature in the form of tubes. Robson acknowledges that synthetic garnierite has been prepared by prior art workers. The nickel chrysotile used in the working examples later in this specification, however, was prepared in accordance with the techniques of Robson and thus the Robson technique is the preferred, although not the only, method of preparing the catalyst for use in the subject invention. In general, this process is to initially synthesize a gel by coprecipitation of the metal oxide or hydroxide with hydrous silica gel in an alkaline medium wherein the pH is above 10, preferably about 12 to 14. The composition of the metal hydroxide layer of the crystal is fixed by selecting the concentration of nickel and ruthenium salts to vary the ratio of nickel to ruthenium as desired. Any water soluble nickel or ruthenium salts can be employed. After the desired gel is produced, it is heated at from about 200.degree. to 350.degree.C., preferably 250.degree. to 275.degree.C., so that the chrysotile product is crystallized from the synthesis gel with rejection of excess water and soluble salts which are removed by filtration and washing. The complex metal silicates as defined above are generally prepared synthetically in hydrated form and are then converted to a dehydrated form by heating prior to use or in situ operation. Since the dehydration reaction is reversible and since water is produced during the methanation reaction, the exact degree of hydration of the catalyst as the reaction proceeds is not known. Thus w in the above formula is defined as ranging from 0 to 4 to indicate that the degree of hydration of the catalyst may vary.
The nickel, ruthenium or mixed nickel-ruthenium chrysotiles are dried to remove surface moisture and may or may not be dehydrated in whole or in part by calcination prior to use. The catalyst also, preferably, undergoes a mild prereduction before use. Calcination is not essential, nor is prereduction with a gas such as hydrogen essential, although varying degrees of calcination and/or prereduction may occur. Since the methanation reaction is operated at elevated temperatures and in the presence of reducing gases, dehydration and reduction of the catalyst will occur during the methanation reaction. Precalcination can suitably occur at temperatures of 300.degree. to 500.degree.C. for 2 to 10 hours. Prereduction using a gas such as H.sub.2 at flow rates of 50 to 500 cc/min can also suitably occur at temperatures of 300.degree. to 500.degree.C. for 2 to 10 hours.
The charge stock for the methanation reaction comprises hydrogen and at least one carbon oxide selected from the group consisting of CO and CO.sub.2 wherein the molar ratio of hydrogen to combined carbon oxides is at least 2:1. Preferably the hydrogen to combined carbon oxides molar ratio is from about 3:1 to 4:1, although ratios to 10:1 to 100:1 to 1000:1 or more can be employed.
Ideally the methanation reaction proceeds in accordance with Equation I below when CO is the reactive carbon oxide employed.